The present invention relates to a wavelength selective optical recording and reproducing device.
FIG. 6 shows a construction of a conventional such device which is disclosed in U.S. Pat. No. 4,101,976, which corresponds to Japanese Patent Publication (Kokoko) No. 58-51355 and FIG. 7 illustrates the wavelength spectrum of a recording medium having recorded information.
In FIG. 6, reference numeral 10 depicts a wavelength variable light source such as semiconductor laser, 11 a controller for varying the wavelength of the light source 10, 12 a collimating lens for converting the light from the light source 10 into a parallel beam, 13 a deflector, 14 an objective lens for condensing the parallel beam to a minute spot and directing it onto a recordable and reproducible medium 15, 16 one arbitrary memory element of the several memory elements on the medium 15 each of which is shown by a circle in this figure, the arbitrary memory element being selected by the deflector 13, and 17 an optical sensor for sensing light passed through the memory element 16.
In operation, light from the light source 10 is converted by the collimating lens 12 into a parallel beam and condensed by the objective lens 14 to a light spot and directed onto the selected memory element 16 on the medium 15. The selection of the memory element can be performed arbitrarily by means of the deflector 13. The principle of the wavelength selective recording and reproducing at the selected memory element 16 will be described with reference to FIG. 7. A waveform (a) in FIG. 7 is an absorption spectrum of the medium prior to wavelength selective recording, which has a broad spectral characteristic. When the medium is irradiated with lights having intensity spectra such as shown by dotted lines, the absorption curve of the medium is reduced at locations corresponding to peaks of the light spectra as shown by a waveform (b) in FIG. 7. Such absorption is called as a "spectral hole". When a spectral hole occurs, it is given the meaning of a "1" memorized on the medium at a corresponding wavelength. No absorption means that "0" is memorized. In order to produce a spectral hole at an arbitrary wavelength, i.e., to write a data "1" at such wavelength, the wavelength of the light source 10 is made to coincide with the wavelength of the spectral hole to be recorded by means of the wavelength controller 11 and to increase the intensity of the light source 10 up to a value necessary to record. In order to read a signal recorded on the medium and having the wavelength selective record spectrum such as shown by the waveform (b) in FIG. 7, it is enough to scan a wavelength from an upper limit A to a lower limit B while the light intensity of the light source 10 is held constant. Since the degree of absorption is reduced at the wavelength of the spectral hole, a spectrum of light intensity such as shown by a waveform (c) in FIG. 7 can be obtained by detecting the transmission of light through the medium 15 by an optical sensor 17. Although the waveform (c) in FIG. 7 shows the wavelength spectrum, it is possible to obtain at the output of the optical detector 17 a signal which is a function of time obtained by scanning through the wavelengths at a constant rate.
Practically, it is possible to provide several thousands of spectral holes in such a broad spectral range as shown by the waveform (a) in FIG. 7. In order to realize the recording of such a large number of spectral holes, it is necessary to exactly control the absolute wavelength of the light source. However, since the wavelength width of a spectral hole is typically on the order of several tens to several hundreds MHz in frequency, it is very difficult to select a wavelength of light corresponding to a frequency unit in such a range, resulting in some error in the reproduced data.